This family is used to handle various data which can be stored on the AIM servers.
Things that can be stored are your buddy list (with groups and buddy comments), your permit and
deny lists, your permit/deny setting, and your visibility setting.
The information is stored as "items." The structure is described here.
Buddy ID#s and group ID#s are arbitrary, though some clients tend to do it sequentially. They cannot, however, be 0x0000, and each ID# must be unique to the item that contains it (ie. two buddies in the same group can not have the same ID#, but two buddies in different groups can have the same ID#. Also, two groups in the list can not have the same ID#)(actually, we're
still not clear as an un-muddied lake on this)(it's possible that the ID#s are some kind of hash of the name).
For @mac.com screen names to show up in your buddy list, you must be using a family version of 3 or later (the most recent is currently 4). If you are using 1 or 2, AIM sends you your buddy list, but where you would normally have "firstname.lastname@example.org," they send "PleaseUpgrade000." The BID and GID are the same, and the list that's actually stored on the server still contains the email@example.com screen name--you just have to sign on with a client supporting a newer version of the SSI protocol.
Alex Yacoub had a big part in writing this, and helping figure out the protocol.